We propose experiments that would examine a fundamental issue in development: whether structure and function in a target structure is determined by intrinsic properties of the target or by its inputs during development. We nave induced, by appropriate surgery in neonatal ferrets, retinal projections into the medial geniculate nucleus (MGN), the principal auditory thalamic nucleus. The MGN with retinal input retains its projections to auditory cortex. We have shown that retinal input to the MGN in "rewired" animals arises from small retinal ganglion cells with slow conduction velocities, that the pathways for visual inputs to the primary auditory cortex (AI) is from the retina via the MGN, that neurons in the MGN and AI have visual receptive fields, and that a map of visual space exists in AI. In the next grant period, we shall examine: (1) the structure of individual retinal ganglion cells that project to the MGN in rewired animals and compare the cells with ganglion cells in the normal retina; (2) the structure of single retinal axon arbors within the MGN in rewired animals, and compare the arbors with those of retinal axons in normal targets; (3) the physiological properties of visual cells and the visual field map in the MGN; (4) the physiological properties of visual cells in AI, and compare their response features with cells in primary visual cortex (VI); (5) the arbors of single thalamocortical axons that project from the MGN to AI in rewired animals; (6) the mapping of thalamocortical fibers in AI after silencing cortical cells by infusion of muscimol; (7) the spatial distribution of thalamocortical excitation and intracortical inhibition in slices of rewired and normal AI; (8) the intracortical restriction of visual receptive fields in AI by microiontophoresis of bicuculline; and (9) whether rewired ferrets perceive visual stimuli presented to the rewired pathway as visual or as auditory. The experiments follow from, and extend, our work in the previous grant period. The studies have implications not only for mechanisms of normal development but also for mechanisms underlying sparing and recovery of function after early trauma to the brain.